It is known in the art to use spacer structures between the cathode and anode plates of a field emission display. The spacer structures maintain the separation between the cathode and the anode plates and prevent implosion of the plates due to the pressure difference between the internal vacuum and the external atmospheric pressure. The spacer structures must also withstand the potential difference between the cathode and the anode.
However, spacers can adversely affect the flow of electrons from the cathode plate toward the anode plate in the vicinity of the spacers. Spacers have been made from dielectric materials, which can withstand the potential difference between the cathode and anode plates and prevent power losses due to electrical conduction between the plates. However, the surfaces of a dielectric spacer can become electrostatically charged by some of the electrons emitted from the cathode plate in the vicinity of the spacers. The charging phenomenon changes the voltage distribution near the spacers from the desired voltage distribution. The change in voltage distribution near the spacers can result in distortion of the electron flow. It can also result in electrical arcing, as between the spacers and the cathode plate.
In a field emission display, this distortion of the electron flow proximate to the spacers can result in distortions in the image produced by the display. In particular, the distortions render the spacers "visible" by producing a dark region in the image at the location of each spacer.
Several prior art spacers attempt to solve the problems associated with spacer charging. For example, it is known in the art to provide a spacer that includes a bulk dielectric material and that has a conductive surface. The conductive surface has a sheet resistance that is low enough to remove accumulated charge by conduction, yet high enough to ameliorate power losses due to electrical current between the anode and cathode plates. The resistive surface can be realized by coating the spacer with a film having the desired resistance. A typical thickness of the resistive coating is less than 1 micrometer.
Many difficulties are encountered with prior art coated spacers. For example, uniformity and reproducibility of very thin resistive films is difficult to realize. Non-uniformity in the thickness of the film ultimately can cause nonuniformity in the output of the device, such as non-uniformity in a display image of a field emission display device. This can be due to, for example, areas or points on the spacers that are capable of becoming charged.
Other disadvantages of coated spacers are the limited electrical ruggedness, mechanical ruggedness, and chemical ruggedness of the resistive coatings. For example, the coatings may not be compatible with other materials within the device or with the vacuum environment. For operating performance to remain constant over the life of the device, the properties of the resistive coating must remain constant. The properties of the coating must not be altered by the impinging electron current, temperature treatments, chemical interactions, etc, during fabrication and during operation of the device.
However, very thin resistive coatings can be sensitive to, for example, the electrical load derived from the current between the anode and the cathode and from impinging charges during the operation of the field emission device. The maximum current density that can be withstood by a very thin resistive film may be too low to accommodate the potential difference between the anode and the cathode. If the current density within the coating exceeds the maximum value for the coating, overheating and material breakdown of the resistive film may occur.
It is also known in the art to provide electrodes on the spacers for the purpose of deflecting or focusing electrons, so that they do not impinge upon the spacer surfaces. This prior art scheme adds complexity and cost to the processes for manufacturing and operating the devices.
Accordingly, there exists a need for an improved field emission device, which has spacers that reduce distortion of electron flow and that do not result in excessive power losses.